Cavities and applicators for microwave heating of materials are typically resonant in operation, since such a condition results in possibilities of achieving a high microwave efficiency. Typical cavity/applicator loads have either a high permittivity such as 10 to 80 for polar liquids and compact food substances, or a lower permittivity but then also a low loss factor and a larger volume, such as in drying operations. In both these cases there is a need for the microwave energy to be reflected and retro-reflected many times in the cavity/applicator in order for a sufficient heating efficiency to be obtained. However, resonant conditions entails a limitation of the frequency bandwidth of proper function.
There are three methods in use to overcome the practical problem of limited resonance frequency bandwidth:                Use of multiple resonances in a comparatively large cavity. At least one resonance will then exist at the operating frequency of the generator such as a magnetron. This type of cavity is easy to use but has the drawback of variable and quite unpredictable heating patterns and microwave efficiency for even slightly different loads, particularly if these are small.        Use of some adjustment means for the resonant frequency in a single mode cavity/applicator. Mechanical means such as movable shorting plungers are cumbersome and require good galvanic contact. A more practical but still mechanically operated device is a non-contacting deflector described in WO-01/62379.        Use of adjustable frequency generators.—Low power semiconductor generators or expensive TWT tubes may be useful, but another problem then occurs: that of the limits of the established ISM bands. For operating frequencies outside these, complicated shielding and filtering is needed.        
If the required frequency variations are within for example the allowed 2400 to 2500 MHz, systems of the third kind above intended for a limited range of load geometries or permittivities may work well. The reduced resonance frequency span in use must then be inherently designed into the microwave applicator.
It may also be possible to achieve negative feedback of the applicator plus load resonant frequency by utilising a combination of applicator cavity and internal load resonant properties. Such systems are then limited to particular and rather narrow load geometries and dielectric properties, such as disclosed in U.S. Pat. No. 5,834,744.